Where to find Cynthia in pokemon dimond

You have to go to Pastoria City & talk to the Galactic Grunt by the Great Marsh (Safari Zone). Then go to Lake Verity; next to Sandgem Town where Cynthia will come & battle the Galactic Grunts. After you fight the Boss she will give you the secret potion, & then she will give you something to give to her grandmother in Celestic Town.